Lithium Secondary Battery With Enhanced Heat-Resistance

ABSTRACT

Disclosed is an electrode whose surface includes an organic/inorganic composite porous coating layer comprising heat-absorbing inorganic particles and a binder polymer, wherein the heat-absorbing inorganic particle is at least one particle selected from the group consisting of antimony-containing compounds, metal hydroxides, guanidine-based compounds, boron-containing compounds and zinc tartrate compounds. A separator using the heat-absorbing inorganic particles as a component for forming or coating the separator, and an electrochemical device including the electrode and/or the separator are also disclosed. The separator using the heat-absorbing inorganic particles as a component for forming or coating the separator can ensure excellent thermal safety and minimizes degradation of the quality of a battery.

TECHNICAL FIELD

The present invention relates to an electrode comprising aheat-absorbing porous coating layer that spontaneously absorbs orconsumes heat generated inside an electrochemical device, a separatorusing heat-absorbing inorganic particles as a component for forming orcoating the separator, and an electrochemical device using the electrodeand/or the separator and having excellent thermal stability.

BACKGROUND ART

Recently, there is increasing interest in energy storage technology.Batteries have been widely used as energy sources in portable phones,camcorders, notebook computers, PCs and electric cars, resulting inintensive research and development into them. In this regard,electrochemical devices are subjects of great interest. Particularly,development of rechargeable secondary batteries has been the focus ofattention. More recently, research and development into an electrode anda battery having a novel design have been conducted in order to improvecapacity density and specific energy thereof.

Among the currently used secondary batteries, lithium secondarybatteries, developed in early 1990's, have drive voltage and energydensity higher than those of conventional batteries using aqueouselectrolytes (such as Ni-MH batteries, Ni—Cd batteries and H₂SO₄—Pbbatteries), and thus they are spotlighted in the field of secondarybatteries. However, lithium secondary batteries have problems related tothe safety, caused by ignition and explosion due to the use of organicelectrolytes, and are manufactured through a complicated process.

Evaluation of and security in safety of batteries are very importantmatters to be considered. Particularly, users should be protected frombeing injured by malfunctioning of batteries. Therefore, safety ofbatteries is strictly restricted in terms of ignition and combustion inbatteries by safety standards. Many attempts have been made to solve theproblem related to the safety of a battery.

More fundamentally, currently available lithium ion batteries andlithium ion polymer batteries use polyolefin-based separators in orderto prevent short circuit between a cathode and an anode. However,because such polyolefin-based separators use a polymer component havinga melting point of 200° C. or less and are subjected to a stretchingstep for controlling their pore sizes and porosities so as to be used asseparators, they have a disadvantage in that they show high heatshrinking property upon exposure to high temperature. In other words,such separators can be shrunk or molten when the temperature of abattery increases due to internal and/or external factors. Therefore,there is a great possibility of a short-circuit between a cathode and ananode that are in direct contact with each other due to shrinking ormelting of separators, resulting in accidents such as ignition andexplosion of a battery caused by rapid emission of electric energy.Therefore, it is necessary to develop a separator that causes no heatshrinking at high temperature.

To solve the above problems related with polyolefin-based separators,many attempts are made to develop an electrolyte using an inorganicmaterial capable of substituting for a conventional separator.

U.S. Pat. No. 6,432,586 discloses a polyolefin-based separator coatedwith an inorganic layer such as calcium carbonate, silica, etc. However,since the composite film still uses a polyolefin-based separator, itcannot provide a significant improvement in the safety of a battery,particularly in terms of the prevention of heat shrinking at hightemperature.

Additionally, Creavis Co. (Germany) have developed an organic/inorganiccomposite separator comprising a non-woven polyester support coated withsilica (SiO₂) or alumina (Al₂O₃). However, in the case of the aboveseparator, the non-woven polyester support cannot provide excellentmechanical and physical properties by nature, and the chemical structureof polyester is liable to electrochemical reactions. Thus, it is thoughtthat the above separator shows many problems in practical use.

Accordingly, there is an imminent need for developing a separator thatcan improve the quality and safety of an electrochemical device, or acomposite electrolyte that also serves as such a separator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a graph showing the endothermic reaction of heat-absorbingparticles (Al(OH)₃) used according to the present invention;

FIG. 2 is a photographic view of the section of an electrode comprisinga heat-absorbing organic/inorganic composite porous coating layeraccording to the present invention, taken by SEM (scanning electronmicroscopy), the electrode including a collector (Al foil), an electrodeactive material layer formed on the collector and a heat-absorbingorganic/inorganic composite porous coating layer formed on the electrodeactive material layer, stacked successively;

FIG. 3 is a photographic view of the surface of a heat-absorbingorganic/inorganic composite porous coating layer formed on the surfaceof a preliminarily formed electrode, taken by SEM;

FIG. 4 is a schematic view of the section of an organic/inorganiccomposite porous separator according to the present invention;

FIG. 5 is a photographic view showing the surface of a porous separatorcomprising heat-absorbing inorganic particles introduced as a componentforming the coating layer, taken by SEM;

FIG. 6 is a graph showing variations in voltage and temperature of alithium secondary battery obtained by using the polyolefin-basedseparator according to Comparative Example 1, after causing anartificial internal short circuit (nail penetration) in the battery;

FIG. 7 is a graph showing variations in voltage and temperature of alithium secondary battery obtained by using a composite separatorcomprising non-heat absorbing inorganic particles introduced theretoaccording to Comparative Example 2, after causing an artificial internalshort circuit (nail penetration) in the battery;

FIG. 8 is a graph showing variations in voltage and temperature of alithium secondary battery obtained by using an organic/inorganiccomposite porous separator comprising heat absorbing inorganic particlesintroduced thereto according to Example 1, after causing an artificialinternal short circuit (nail penetration) in the battery;

FIG. 9 is a photographic view showing the organic/inorganic compositeporous separator and a currently used polyolefin-based separator afterstoring them at room temperature (a) and 150° C. for 1 hour (b),respectively.

DISCLOSURE Technical Problem

The inventors of the present invention have found that when conventionalinorganic particles are used as a component for forming or coating aseparator, it is not possible to fundamentally solve the problem of heatenergy generated rapidly upon an internal short circuit between acathode and an anode caused by external or internal impacts, resultingin dangerous accidents such as ignition or explosion with time or underthe application of a secondary impact, even though no heat shrinkingoccurs even under high temperature conditions in an electrochemicaldevice.

Considering this, according to the present invention, heat-absorbinginorganic particles that absorb or consume heat generated rapidly insidean electrochemical device are used as a component for forming or coatinga separator.

Technical Solution

An aspect of the present invention provides an electrode whose surfaceincludes an organic/inorganic composite porous coating layer comprisingheat-absorbing inorganic particles and a binder polymer, wherein theheat-absorbing inorganic particle is at least one particle selected fromthe group consisting of antimony-containing compounds, metal hydroxides,guanidine-based compounds, boron-containing compounds and zinc tartratecompounds. There is also provided an electrochemical device, preferablya lithium secondary battery, comprising the above electrode.

Another aspect of the present invention provides a separator comprisingheat-absorbing particles that absorb heat energy generated at atemperature (T) higher than a normal drive temperature of anelectrochemical device to be pyrolyzed, or consume such heat energy, asa component for forming or coating the separator. There is also providedan electrochemical device, preferably a lithium secondary battery,comprising the above separator.

Hereinafter, the present invention will be explained in more detail.

The present invention is characterized by using heat-absorbing inorganicparticles as a component for forming or coating a separator that servesto prevent a cathode and an anode from being in direct contact with eachother and to provide a pathway through which lithium ions pass.

Herein, the separator according to the present invention may be realizedto have various forms, including a free-standing separator or a coatinglayer formed on the surface of a substrate. For example, the separatormay be provided as a free-standing separator comprising inorganicparticles and a binder polymer, wherein interstitial volumes among theinorganic particles form a pore structure; a separator comprising aporous substrate and an organic/composite porous coating layer formed onthe porous substrate, the coating layer including inorganic particlesand a binder polymer; or as a monolithic composite electrode functioningnot only as an electrode but also as a separator, the electrodeincluding an electrode substrate and an organic/inorganic compositeporous coating layer serving as a separator and formed on the electrodesubstrate. However, the separator according to the present invention isnot limited thereto.

The heat-absorbing inorganic particles are those that can absorb orconsume heat as soon as it is generated inside an electrochemicaldevice, preferably a battery, and cause spontaneous pyrolysis or producea new product by using the heat energy absorbed thereby.

The heat-absorbing inorganic particles introduced into the separatorinhibit generation of an internal short circuit by preventing a cathodeand an anode from being in direct contact with each other. Contrary toconventional non-heat absorbing inorganic particles, the heat-absorbinginorganic particles can inhibit rapid heat emission, even when a thermalrunaway phenomenon or internal short circuit occurs due to external orinternal factors, and thus can fundamentally prevent ignition andexplosion of a battery. Additionally, since the inorganic particles haveheat resistance, no heat shrinking occurs at high temperature contraryto a conventional polyolefin-based separator (m.p.: 120˜140° C.).

Further, because the heat-absorbing inorganic particles are used as acomponent not for forming an electrode, in which electrochemicalreactions occur, but for forming and/or coating a separator, there is nodrop in capacity of a battery caused by the use of such inorganicparticles as a material for forming an electrode.

<Heat-Absorbing Inorganic Particles>

There are no particular limitations in composition, shape, content, etc.of the heat-absorbing inorganic particles used as a component forforming and/or coating a separator, as long as the particles can absorbor consume heat generated abnormally inside an electrochemical device.

Preferably, even if the heat-absorbing inorganic particles arepyrolyzed, they are decomposed into at least one material havingapparent physical properties (including particle diameter or shape)similar to those of the original heat-absorbing inorganic particles.

The heat-absorbing inorganic particles absorb heat energy preferably ata temperature (T) higher than a normal driving temperature of anelectrochemical device, wherein the normal driving temperature is atmost about 90° C. For example, in the case of an internal short circuit,a rapid local heat emission occurs at a temperature of 400˜500° C.,resulting in shrinking of a separator. At this time, the separatorcomprising heat-absorbing inorganic particles introduced theretoinhibits such rapid local heat emission, and thus imparts improvedsafety to an electrochemical device. In fact, it could be seen from theexperimental example performed by the inventors of the present inventionthat when an internal short circuit occurs in a battery, the separatorcomprising heat-absorbing particles according to the present inventionprevents the battery from being heated to a temperature of 100° C. orhigher, and thus ensures excellent thermal safety of the battery (seeFIG. 8).

Non-limiting examples of the heat-absorbing inorganic particles includeantimony-containing compounds, metal hydroxides, guanidine-basedcompounds, boron-containing compounds, zinc tartrate compounds, mixturesthereof, or the like.

Non-limiting examples of the antimony-containing compounds includeantimony trioxide (Sb₂O₃), antimony tetraoxide (Sb₂O₄), antimonypentaoxide (Sb₂O₅) or a mixture thereof. Non-limiting examples of themetal hydroxides include aluminum hydroxide (Al(OH)₃), magnesiumhydroxide (Mg(OH)₂), or a mixture thereof. Non-limiting examples of theguanidine-based compounds include guanidine nitrate, guanidinesulfaminate, guanidine phosphate, guanyl urea phosphate or a mixturethereof. Non-limiting examples of the boron-containing compounds includeH₃BO₃, HBO₂ or a mixture thereof. Non-limiting examples of the zinctartrate compounds include Zn₂SnO₄, ZnSnO₃ (zinc stannate, ZS),ZnSn(OH)₆ (zinc hydroxyl stannate, ZHS) or a mixture thereof.

The above zinc tartrate compounds are pyrolyzed at about 200° C. by anendothermic reaction. When zinc tartrate compounds experience anendothermic reaction, they absorb abnormal heat energy inside a battery,and thus can inhibit a series of exothermic reactions in the battery.Additionally, because the products produced by the zinc tartratecompounds have excellent flame resistance, the zinc tartrate compoundsserve to inhibit combustion so that a thermal runaway phenomenonoccurring in an electrochemical device cannot proceed to ignition andexplosion.

Additionally, aluminum hydroxide, which is a kind of metal hydroxide, isdecomposed into Al₂O₃ and water (H₂O) by absorbing heat at a temperatureof 200° C. or higher. At this time, aluminum hydroxide absorbs heatenergy of about 1000 J/g (see the following Reaction Scheme 1 and FIG.1). Further, magnesium hydroxide also shows heat-absorbing property withheat energy absorption of about 1300 J/g (see the following ReactionScheme 2). Therefore, when heat energy accumulated in the inorganicparticles corresponds to the above-mentioned heat energy values or whenheat emission occurs to a level corresponding to the above-mentionedheat energy values, endothermic reactions occur immediately so as toimprove the safety of an electrochemical device.

2Al(OH)₃→Al₂O₃+3H₂O (200° C. or higher) ΔH=−1051 J/g  [Reaction Scheme1]

Mg(OH)₂→+MgO+H₂O (340° C. or higher) ΔH=−1316 J/g  [Reaction Scheme 2]

In addition, as shown in the following Reaction Schemes 3 and 4, boroncompounds are pyrolyzed at a temperature of 130° C. or higher andswelled in H₂O, so that they exist as molten compounds having flameresistance.

2H₃BO₃→2HBO₂+2H₂O (130˜200° C.)  [Reaction Scheme 3]

2HBO₂→B₂O₃+H₂O (260˜270° C.)  [Reaction Scheme 4]

Also, antimony-containing compounds are pyrolyzed and absorb the heatgenerated inside an electrochemical device, thereby improving the safetyof an electrochemical device.

Besides the above-mentioned inorganic particles, other compounds thatabsorb the heat generated in an electrochemical device to be pyrolyzedor to produce new compounds are also included in the scope of thepresent invention. Additionally, inorganic flame resistant materials ororganic flame resistant materials known to those skilled in the art maybe used in combination with the heat-absorbing inorganic particles.Particularly, a combination of the heat-absorbing inorganic particleswith an organic flame resistant material, such as a halide, furtherimproves the safety of an electrochemical device.

The heat-absorbing inorganic particles according to the presentinvention can serve not only to form pores by the interstitial volumesamong the interconnected inorganic particles but also to maintain thephysical shape of a separator as a kind of spacer.

According to the present invention, inorganic particles having a highdielectric constant and/or low density may be used optionally togetherwith the heat-absorbing inorganic particles. When the inorganicparticles have a high dielectric constant, they can increase thedissociation degree of lithium ions in a liquid electrolyte. Preferably,the inorganic particles have a dielectric constant of 5 or more.Non-limiting examples of such inorganic particles having a dielectricconstant of 5 or more include SrTiO₃, SnO₂, CeO₂, MgO, NiO, ZnO, Y₂O₃,ZrO₂, Al₂O₃, TiO₂, BaTiO₃ or a mixture thereof.

Although there is no particular limitation in the size of theheat-absorbing inorganic particles, the inorganic particles preferablyhave a size of 0.001˜10 μm in order to form a separator having a uniformthickness and to, provide an adequate porosity. If the size is less than0.001 μm, the inorganic particles have poor dispersibility so that thestructure and physical properties of the organic/inorganic compositeporous separator cannot be controlled with ease. If the size is greaterthan 10 μm, the resultant organic/inorganic composite porous separatorhas an increased thickness under the same solid content, resulting indegradation in mechanical properties. Furthermore, such excessivelylarge pores may increase a possibility of an internal short circuitbeing generated during repeated charge/discharge cycles.

<Binder Polymer>

One component for forming or coating the separator according to thepresent invention is a binder polymer currently used in the art.

The binder polymers preferably have a glass transition temperature(T_(g)) as low as possible, more preferably T_(g) of between −200° C.and 200° C., because they can improve mechanical properties such asflexibility and elasticity of a final coating layer.

When the binder polymer has ion conductivity, it can further improve theperformance of an electrochemical device. Therefore, the binder polymerpreferably has a dielectric constant as high as possible. In practice,because the dissociation degree of a salt in an electrolyte depends onthe dielectric constant of a solvent used in the electrolyte, thepolymer having a higher dielectric constant can increase thedissociation degree of a salt in the electrolyte used in the presentinvention. The dielectric constant of the polymer may range from 1.0 to100 (as measured at a frequency of 1 kHz), and is preferably 10 or more.

Further, when using a binder polymer having a high degree of swellingwith an electrolyte, the electrolyte can infiltrate into the polymer,and thus the binder polymer can impart electrolyte ion conductivity tothe organic/inorganic composite porous separator or can improve theelectrolyte ion conductivity. More particularly, the surface of theinorganic particles used according to the prior art serves as aresistance layer interrupting lithium ion movements. However, when abinder polymer has a high degree of swelling with an electrolyte on thesurface of the porous inorganic particles and/or in the pores formed onthe interstitial volumes among the inorganic particles, interfacialresistance occurring between the inorganic particles and the electrolytedecreases so that solvated lithium ions can be drawn and moved towardthe inside of the pores. Such improved lithium ion conduction canactivate electrochemical reactions in a battery and can improve thequality of a battery. In addition to the above, when the binder polymeris a polymer that can be gelled when swelled with an electrolyte, thepolymer can react with an electrolyte injected subsequently into abattery, and thus can be gelled to form a gel type organic/inorganiccomposite electrolyte. Therefore, it is preferable to use a polymerhaving a solubility parameter of between 15 and 45 MPa^(1/2), morepreferably of between 15 and 25 MPa^(1/2), and between 30 and 45MPa^(1/2). If the binder polymer has a solubility parameter of less than15 Mpa^(1/2) or greater than 45 Mpa^(1/2), it is difficult for thebinder polymer to be swelled with a conventional liquid electrolyte fora battery.

Non-limiting examples of the binder polymer that may be used in thepresent invention include polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinyl pyrrolidone, polyvinyl acetate,polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethylsucrose, pullulan, carboxymethyl cellulose,acrylonitrile-styrene-butadiene copolymer, polyimide,polyacarylonitrile-co-styrene, gelatine, polyethylene glycol,polyethylene glycol dimethyl ether, glyme, polyvinylidene fluoride ormixtures thereof. Other materials may be used alone or in combination,as long as they satisfy the above characteristics.

<Organic/Inorganic Composite Porous Separator>

The organic/inorganic composite porous separator comprisingheat-absorbing particles according to the present invention may bepresent as a free-standing separator, or as a coating layer formed on asubstrate and capable of functioning as a separator.

The organic/inorganic composite porous separator may have any one of thefollowing three types of structures, but is not limited thereto.

{circle around (1)} In the first embodiment, the organic/inorganiccomposite porous separator may form a free-standing typeorganic/inorganic composite porous separator merely by using a mixtureof heat-absorbing inorganic particles and a binder polymer.

In the free-standing organic/inorganic composite porous separator,interstitial volumes among the heat-absorbing inorganic particles thatserve not only as a support but also as a spacer for a pore structurehaving a uniform pore size and porosity. More preferably, thefree-standing organic/inorganic composite porous separator includesheat-absorbing particles and a binder polymer coating layer partially ortotally formed on the surface of the inorganic particles, wherein theinorganic particles are interconnected among themselves and are fixed bythe binder polymer, and the interstitial volumes among theheat-absorbing inorganic particles form a pore structure.

{circle around (2)} In the second embodiment, the organic/inorganiccomposite porous separator may be provided by coating a porous separatorsubstrate having pores with the above mixture so as to form anorganic/inorganic composite porous coating layer capable of functioningas a separator on the surface of the porous substrate and/or in thepores of the substrate (see FIG. 4).

In the organic/inorganic composite porous separator according to thepresent invention, there is no particular limitation in the substrate aslong as the substrate is a porous separator substrate having pores. Forexample, the porous separator substrate may include a polyolefin-basedseparator currently used in the art or a heat resistant porous substratehaving a melting point of 200° C. or higher. Particularly, when using aheat resistant porous substrate, it is possible to prevent a separatorfrom shrinking by external and/or internal heat application, and thus toensure the thermal safety of the organic/inorganic composite porousseparator.

Non-limiting examples of the porous separator substrate that may be usedin the present invention include high density polyethylene, low densitypolyethylene, linear low density polyethylene, ultrahigh molecularweight polyethylene, polypropylene, polyethylene terephthalate,polybutylene terephthalate, polyester, polyacetal, polyamide,polycarbonate, polyimide, polyetheretherketone, polyethersulfone,polyphenylene oxide, polyphenylenesulfide, polyethylene naphthalene orcombinations thereof. Also, other heat resistant engineering plasticsmay be used with no particular limitation.

Although there is no particular limitation in the thickness of theporous separator substrate, the substrate preferably has a thicknessbetween 1 μm and 100 μm, more preferably between 5 μm and 50 μm. If thesubstrate has a thickness less than 1 μm, it is difficult to maintainmechanical properties. If the substrate has a thickness greater than 100μm, it may function as a resistance layer.

Although there is no particular limitation in the pore size and porosityof the porous separator substrate, the substrate preferably has aporosity of between 5% and 95%. The pore size (diameter) preferablyranges from 0.01 μm to 50 μm, more preferably from 0.1 μm to 20 μm. Whenthe pore size and porosity are less than 0.01 μm and 5%, respectively,the substrate may function as a resistance layer. When the pore size andporosity are greater than 50 μm and 95%, respectively, it is difficultto maintain mechanical properties.

Therefore, the organic/inorganic composite porous separator according tothe present invention comprises a porous substrate having pores and anorganic/inorganic composite porous coating layer formed on the surfaceor in the pores of the substrate by using a mixture of heat-absorbinginorganic particles and a binder polymer, wherein the heat-absorbinginorganic particles are interconnected among themselves and fixed by thebinder polymer, and the interstitial volumes among the heat-absorbinginorganic particles form a pore structure.

{circle around (3)} In the third embodiment, the above mixture is coatedonto a preliminarily formed cathode and/or anode to form anorganic/inorganic composite porous separator directly on an electrode.In this case, the organic/inorganic composite porous separator is formedintegrally with an electrode capable of reversible lithiumintercalation/deintercalation.

The composite electrode comprising the organic/inorganic compositeporous separator thereon according to the present invention is obtainedby coating a mixture of heat-absorbing inorganic particles and a binderpolymer onto the surface of an electrode comprising a collector andelectrode active material particles bound to the collector while forminga pore structure, wherein the heat-absorbing inorganic particles areinterconnected among themselves and fixed by the binder polymer, and theinterstitial volumes among the heat-absorbing inorganic particles form apore structure.

Since the electrode comprising the organic/inorganic composite porousseparator according to the present invention is formed by coating theseparator directly onto the surface of an electrode substrate includingelectrode active material particles bound to a collector while forming apore structure, the electrode active material layer and theorganic/inorganic composite porous separator can be anchored to eachother and firmly bound to each other physically and organically.Therefore, problems related to mechanical properties such as brittlenesscan be improved by virtue of an excellent interfacial adhesion betweenthe electrode substrate and the organic/inorganic composite porouscoating layer.

The organic/inorganic composite porous separator that can be realized invarious forms as described above is characterized by comprising a porestructure having a uniform pore size and porosity.

First of all, in the free-standing organic/inorganic composite porousseparator, interstitial volumes among the inorganic particlesfunctioning not only as a support but also as a spacer form a porestructure. Additionally, the organic/inorganic composite porousseparator formed by coating the above mixture onto a porous substratehas pore structures not only in the porous substrate itself, but also inthe coating layer due to the interstitial volumes among the inorganicparticles formed on the substrate. Further, the composite electrodeformed by coating an electrode substrate with the above mixture includesa pore structure having a uniform pore size and porosity due to theinterstitial volumes among the heat-absorbing particles in addition tothe pore structure formed by electrode active material particles in theelectrode. Therefore, it is possible to significantly reduce interfacialresistance upon the injection of an electrolyte, to increase the spaceto be impregnated with an electrolyte by virtue of the pore structurehaving a uniform size, and to facilitate lithium ion conduction. As aresult, the pore structure unique to the organic/inorganic compositeporous separator according to the present invention can minimizedegradation of the quality of a battery (see FIGS. 2 and 3 and Table 1).

In the organic/inorganic composite porous separators according to theabove preferred embodiments of the present invention, the heat-absorbinginorganic particles are present preferably in a ratio of 10˜99:1˜90(heat-absorbing particles:binder polymer, on the weight basis), morepreferably in a ratio of 50˜95:5˜50. If the content of theheat-absorbing inorganic particles is excessively low, the binderpolymer is present in such a large amount that the interstitial volumesformed among the inorganic particles are decreased, and thus the poresize and porosity are reduced, resulting in degradation in the qualityof a finished battery. If the content of the heat-absorbing inorganicparticles is excessively high, the polymer content is too low to providesufficient adhesion among the inorganic particles, resulting indegradation in mechanical properties of a finally formedorganic/inorganic composite porous coating layer.

There is no particular limitation in the thickness of theorganic/inorganic composite porous separator. Also, when theorganic/inorganic composite porous separator is formed integrally on thesurface of an electrode, it is possible to independently control thethickness of each separator on a cathode and on an anode. According tothe present invention, it is preferable to control the thickness of theseparator in a range of 1˜100 μm, more preferably of 1˜30 μm, in orderto reduce the internal resistance of a battery.

Additionally, the organic/inorganic composite porous separatorpreferably has a pore size of 0.001˜10 μm and a porosity of 5˜95%, butis not limited thereto.

The organic/inorganic composite porous separator according to thepresent invention may be obtained by using a conventional method knownto those skilled in the art. In a preferred embodiment of the method,the heat-absorbing inorganic particles are added to and mixed with apolymer solution in which a binder is dissolved, and then the mixture iscoated and dried on a substrate.

Herein, when a porous substrate having pores and a preliminarily formedelectrode are used as the substrates, the organic/inorganic compositeporous separators according to the second embodiment and the thirdembodiment can be provided, respectively. Also, when the mixture iscoated on a substrate and then the substrate is removed, the abovefree-standing type organic/inorganic composite porous separator can beprovided.

Also, as the solvent for dissolving the binder polymer, it is possibleto use any solvent known to those skilled in the art with no particularlimitation. A solvent having a solubility parameter similar to thesolubility parameter of the binder polymer and a low boiling point ispreferred.

It is preferable to perform a step of pulverizing inorganic particlesafter adding the heat-absorbing inorganic particles to the preformedbinder polymer solution. Conventional pulverization methods, preferablya method using a ball mill, may be used.

According to the present invention, in order to control the pore size,porosity and thickness of an organic/inorganic composite porous coatinglayer to be formed finally, various factors for controlling pores of thecoating layer, including pore size, porosity, dimension (particlediameter) and content of the porous inorganic particles and compositionof the porous inorganic particles and the binder polymer, may beadjusted as necessary.

For example, as the weight ratio (I/P) of the heat-absorbing inorganicparticles (I) to the polymer (P) increases, pore forming capability ofthe organic/inorganic composite porous coating layer increases due to anincrease in interstitial volumes among the inorganic particles.Therefore, pore size and porosity increase in the finally formedorganic/inorganic composite porous coating layer. On the other hand, thethickness of the organic/inorganic composite porous separator increasesunder the same solid content (weight of the inorganic particles+weightof the binder polymer). Additionally, as the size (particle diameter) ofthe inorganic particles increases, interstitial distance among theinorganic particles increases, thereby increasing the pore size.

The mixture of the heat-absorbing inorganic particles with the binderpolymer obtained as described above is coated on a substrate by usingany method known to one skilled in the art, including dip coating, diecoating, roll coating, comma coating or combinations thereof. Then, thecoating layer is dried to provide the organic/inorganic composite porousseparator according to the present invention.

The organic/inorganic composite porous separator obtained as describedabove according to the present invention may be used as a separator foran electrochemical device, preferably a lithium secondary battery. Inthe organic/inorganic composite porous separator, the heat-absorbinginorganic particles included therein or coated thereon can inhibit theseparator from shrinking or melting at high temperature.

<Electrochemical Device>

Further, the present invention provides an electrochemical devicecomprising a cathode, an anode and an electrolyte, the electrochemicaldevice comprising an electrode having the organic/inorganic compositeporous separator, a separator comprising heat-absorbing inorganicparticles introduced thereto, or both.

Such electrochemical devices include any devices in whichelectrochemical reactions occur, and particular examples thereof includeall kinds of primary batteries, secondary batteries, fuel cells, solarcells or capacitors. Particularly, the electrochemical device is alithium secondary battery including a lithium metal secondary battery,lithium ion secondary battery, lithium polymer secondary battery orlithium ion polymer secondary battery.

The electrochemical device may be manufactured by a conventional methodknown to one skilled in the art. In one embodiment of the method formanufacturing the electrochemical device, an electrode assembly having acathode and an anode is formed and an electrolyte is injected into theelectrode assembly. Herein, the above-described organic/inorganiccomposite porous separator may be interposed between both electrodes toprovide the electrochemical device. Also, when using a monolithiccomposite electrode capable of functioning also as a separator by virtueof the organic/inorganic composite porous separator and, theelectrochemical device is assembled by using a cathode and an anode.Therefore it is possible to simplify the process for manufacturing anelectrochemical device.

There are no particular limitations in the cathode, anode andelectrolyte that may be applied in combination with theorganic/inorganic composite porous separator according to the presentinvention. Any cathodes, anodes and electrolytes generally used inconventional electrochemical devices may be used.

Also, the electrochemical device according to the present invention mayfurther comprise a microporous separator, such as a polyolefin-basedseparator, in addition to the organic/inorganic composite porousseparator according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

Example 1 Manufacture of Anode

To N-methyl-2-pyrrolidone (NMP) as a solvent, 96 wt % of carbon powderas an anode active material, 3 wt % of polyvinylidene fluoride (PVDF) asa binder and 1 wt % of carbon black as a conductive agent were added toform mixed slurry for an anode. The slurry was coated on Cu foil havinga thickness of about 10 μm as an anode collector, and then dried to forman anode. Then, the anode was subjected to roll press.

(Manufacture of Cathode)

To N-methyl-2-pyrrolidone (NMP) as a solvent, 92 wt % of lithium cobaltcomposite oxide (LiCoO₂) as a cathode active material, 4 wt % of carbonblack as a conductive agent and 4 wt % of PVDF as a binder were added toform slurry for a cathode. The slurry was coated on Al foil having athickness of about 20 μm as a cathode collector, and then dried to forma cathode. Then, the cathode was subjected to roll press.

(Electrode Surface Coating)

About 5 wt % of PVdF-CTFE polymer (polyvinylidenefluoride-chlorotrifluoroethylene copolymer) having a solubilityparameter of 20˜25 MPa^(1/2) or PVdF-HFP (polyvinylidenefluoride-hexyluoropropylene) polymer having a solubility parameter of22˜30 MPa^(1/2) was added to acetone and dissolved therein at 50° C. forabout 12 hours or more to provide a polymer solution. To the preformedpolymer solution, aluminum hydroxide (Al(OH)₃) powder was added in anamount of 20 wt % on the solid content basis, and the aluminum hydroxidepowder was pulverized and dispersed by using a ball mill for 12 hours ormore to provide slurry. In the slurry, particle diameter of the aluminumhydroxide particles may be controlled according to the size (particlesize) of the beads and the ball milling time. In this example, thealuminum hydroxide particles are pulverized into a size of about 800 nmto provide slurry. Then, the slurry was coated onto surfaces of thepreliminarily formed cathode and anode to a thickness of about 15 μm viaa dip coating process. After calculating average pore size and porosityof the resultant organic/inorganic composite porous coating layer fromthe SEM photograph as shown in FIG. 3, the coating layer has an averagepore size and porosity of 0.3 μm and 45%, respectively.

The anode and the cathode obtained as described above were stacked toprovide an electrode assembly using no conventional polyolefin-basedseparator. Then, an electrolyte comprising 1M lithiumhexafluorophosphate (LiPF₆) in ethylene carbonate (EC), propylenecarbonate (PC) and diethyl carbonate (DEC) in a weight ratio of 30/20/50(EC/PC/DEC) was injected thereto to provide a lithium secondary battery.

Example 2

Example 1 was repeated to provide an organic/inorganic composite porousseparator and a battery including the same, except that a mixture of thepolymer and aluminum hydroxide (Al(OH)₃) powder was coated onto apolyethylene separator having a thickness of about 18 μm (porosity 45%)instead of the preliminarily formed electrode.

After measuring the average pore size and porosity by using aporosimeter, the organic/inorganic composite porous separator had anaverage pore size of 0.4 μm and a porosity of 55%. FIG. 4 shows thestructure of the organic/inorganic composite porous separator.

Example 3

Example 1 was repeated to provide an organic/inorganic composite porousseparator and a battery including the same, except that a mixture of thepolymer and aluminum hydroxide (Al(OH)₃) powder was coated onto a teflonsheet substrate instead of the preliminarily formed electrode, thesolvent was dried, and then the teflon sheet was removed.

Comparative Example 1

Example 1 was repeated to provide a lithium secondary battery, exceptthat a conventional polyethylene (PE) separator known to those skilledin the art was used.

Comparative Example 2

Example 1 was repeated to provide an electrode and a lithium secondarybattery including the same electrode, except that alumina (Al₂O₃)particles as non-heat absorbing inorganic particles were used instead ofaluminum hydroxide (Al(OH)₃) particles.

Experimental Example 1 Surface Analysis of Organic/Inorganic CompositePorous Separator

The following test was performed to analyze the surface of theorganic/inorganic composite porous separator according to the presentinvention.

The sample used in this test was the organic/inorganic composite porousseparators comprising heat-absorbing inorganic particles (Al(OH)₃)according to Examples 1 and 2.

When analyzed by using a Scanning Electron Microscope (SEM), theorganic/inorganic composite porous separator according to Example 1showed uniform pore structures formed in the electrode substrate itself(see FIG. 2) as well as in the organic/inorganic composite porouscoating layer using the heat-absorbing inorganic particles (see FIG. 3).Similarly, the organic/inorganic composite porous separator according toExample 2 showed a uniform pore structure formed in theorganic/inorganic composite porous coating layer using theheat-absorbing inorganic particles (see FIG. 5).

Experimental Example 2 Evaluation for Safety of Lithium SecondaryBattery

The following test was performed to evaluate the safety of a lithiumsecondary battery comprising the organic/inorganic composite porousseparator according to the present invention.

2-1. Nail Penetration Test

The lithium secondary battery including the organic/inorganic compositeporous separator comprising heat-absorbing inorganic particles accordingto Example 1 was used as a sample. As controls, the lithium secondarybatteries using the polyolefin-based separator according to ComparativeExample 1 and the organic/inorganic composite porous separatorcomprising conventional non-heat absorbing inorganic particles accordingto Comparative Example 2 were used. The batteries were subjected to thefollowing nail penetration test.

The nail penetration test is a test for observing whether a batterycauses ignition and explosion or not, after an artificial internal shortcircuit is generated by causing a sharp needle-like object to penetrateinto the battery at a constant speed so that a cathode and an anode arein direct contact with each other.

After the test, the battery using the polyolefin-based separatoraccording to Comparative Example 1 showed a rapid drop in voltage to 0due to the generation of an internal short circuit in the battery andignited due to an increase in internal temperature of the battery (seeFIG. 6). Additionally, the battery using the organic/inorganic compositeporous separator comprising non-heat absorbing inorganic particlesaccording to Comparative Example 2 did not ignite, but showed anincrease in surface temperature to 125° C. (see FIG. 7). Such anexcessive increase in temperature of a battery significantly increases apossibility of ignition or explosion with time.

On the contrary, the battery using the organic/inorganic compositeporous separator comprising heat absorbing inorganic particles accordingto Example 1 did not ignite and showed an increase in surfacetemperature merely to 100° C. This indicates that use of theheat-absorbing inorganic particles ensures the safety of a battery (seeFIG. 8).

2-2. Hot Box Test

Each battery was stored at a high temperature of 150° C. for 1 hour.Then, the condition of each battery was observed.

After the test, the battery using a conventional polyolefin-basedseparator according to Comparative Example 1 showed severe heatshrinking and molten and broken after storing it at a temperature of150° C. for 1 hour. Contrary to this, the organic/inorganic compositeporous separator comprising heat-absorbing inorganic particles accordingto Example 2 experienced no change even after storing it at a hightemperature of 150° C., and showed excellent thermal stability (see FIG.9).

Experimental Example 3 Evaluation for Quality of Lithium SecondaryBattery

The following test was performed in order to evaluate C-ratecharacteristics of the lithium secondary battery comprising theorganic/inorganic composite porous separator according to the presentinvention.

The lithium secondary battery using the organic/inorganic compositeporous separator comprising heat-absorbing particles according toExample 1, the lithium secondary battery using the polyolefin-basedseparator according to Comparative Example 1 and the lithium secondarybattery using the organic/inorganic composite porous separator accordingto Comparative Example 2 were used in this test. Each battery having acapacity of 2.4 Ah was subjected to cycling at a discharge rate of 0.2C, 0.5 C, 1 C, 1.5 C and 2 C. The following Table 1 shows the dischargecapacity of each battery, the capacity being expressed on the basis ofC-rate characteristics.

After the test, it can be seen that the lithium secondary battery usingthe organic/inorganic composite porous separator comprisingheat-absorbing inorganic particles according to Example 1 is comparableto the battery using the conventional polyolefin-based separator interms of C-rate characteristics under a discharge rate up to 2 C (seeTable 1).

TABLE 1 Capacity 0.2 C 0.5 C 1.0 C 1.5 C 2.0 C Comp. mAh 2408 2377 23202293 2240 Ex. 1 % 100.0% 98.7% 96.3% 95.2% 93.0% Comp. mAh 2406 23732316 2279 2224 Ex. 2 % 100.0% 98.6% 96.3% 94.7% 92.5% Ex. 1 mAh 24102372 2325 2295 2243 % 100.0% 98.4% 96.5% 95.2% 93.1%

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, the organic/inorganic compositeporous separator, using heat-absorbing inorganic particles that absorbor consume heat energy generated in an electrochemical device as acomponent for forming or coating the separator, consumes heat energygenerated upon rapid heating caused by external or internal factors, andthus can ensure excellent thermal safety of the electrochemical device.

Although several preferred embodiments of the present invention havebeen described for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. An electrode whose surface includes an organic/inorganic compositeporous coating layer comprising heat-absorbing inorganic particles and abinder polymer, wherein the heat-absorbing inorganic particle is atleast one particle selected from the group consisting ofantimony-containing compounds, metal hydroxides, guanidine-basedcompounds, boron-containing compounds and zinc tartrate compounds. 2.The electrode as claimed in claim 1, wherein the heat-absorbinginorganic particles absorb heat energy generated at a temperature (T)higher than a normal drive temperature of an electrochemical device sothat they are pyrolyzed, or consume the heat energy.
 3. The electrode asclaimed in claim 1, wherein the antimony-containing compound is selectedfrom antimony trioxide (Sb₂O₃), antimony tetraoxide (Sb₂O₄) and antimonypentaoxide (Sb₂O₅); the metal hydroxide is selected from aluminumhydroxide (Al(OH)₃) and magnesium hydroxide (Mg(OH)₂); theguanidine-based compound is selected from guanidine nitrate, guanidinesulfaminate, guanidine phosphate and guanyl urea phosphate; theboron-containing compound is selected from H₃BO₃ and HBO₂; and the zinctartrate compound is selected from Zn₂SnO₄, ZnSnO₃ and ZnSn(OH)₆.
 4. Theelectrode as claimed in claim 1, wherein the organic/inorganic compositeporous coating layer is formed by coating a mixture of heat-absorbinginorganic particles and a binder polymer onto the surface of anelectrode comprising a collector and electrode active material particlesbound to the collector while forming a pore structure, theheat-absorbing inorganic particles are interconnected among themselvesand fixed by the binder polymer, and interstitial volumes among theheat-absorbing inorganic particles form a pore structure.
 5. Theelectrode as claimed in claim 1, wherein the organic/inorganic compositeporous coating layer functions as a separator that prevents a cathodeand an anode from being in direct contact with each other and allowslithium ions (Li⁺) to pass therethrough.
 6. The electrode as claimed inclaim 1, wherein the binder polymer has a solubility parameter of 15˜45MPa^(1/2).
 7. The electrode as claimed in claim 1, wherein theheat-absorbing inorganic particles and the binder polymer are used in aweight ratio of 10:90˜99:1.
 8. The electrode as claimed in claim 1,wherein the organic/inorganic composite porous coating layer has athickness of 1˜100 μm.
 9. A separator comprising heat-absorbingparticles which absorb heat energy generated at a temperature (T) higherthan a normal drive temperature of an electrochemical device so thatthey are pyrolyzed, or, consume the heat energy, as a component forforming or coating the separator.
 10. The separator as claimed in claim9, wherein the heat-absorbing inorganic particle is at least oneparticle selected from the group consisting of antimony-containingcompounds, metal hydroxides, guanidine-based compounds, boron-containingcompounds and zinc tartrate compounds.
 11. The separator as claimed inclaim 9, which is any one of the following separators: (a) a separatorcomprising heat-absorbing inorganic particles and a binder polymer,wherein the heat-absorbing inorganic particles are interconnected amongthemselves and fixed by the binder polymer and interstitial volumesamong the heat-absorbing inorganic particles form a pore structure; and(b) a separator comprising a porous substrate; and an organic/inorganiccomposite porous coating layer totally or partially formed on thesurface or in the pores of the substrate by using a mixture ofheat-absorbing inorganic particles and a binder polymer, wherein theheat-absorbing inorganic particles are interconnected among themselvesand fixed by the binder polymer, and the interstitial volumes among theheat-absorbing inorganic particles form a pore structure.
 12. Theseparator as claimed in claim 9, wherein the heat-absorbing inorganicparticles prevent the separator from shrinking or melting.
 13. Theseparator as claimed in claim 11, wherein the porous substrate is formedof a polyolefin-based polymer or a polymer having a melting point of200° C. or higher.
 14. An electrochemical device comprising a cathode,an anode, a separator and an electrolyte, which comprises the electrodeas defined in claim
 1. 15. The electrochemical device as claimed inclaim 14, wherein the heat-absorbing inorganic particles contained inthe electrode inhibits the electrochemical device from undergoing rapidheat emission and ignition caused by an internal short circuit.
 16. Theelectrochemical device as claimed in claim 14, which further comprises amicroporous separator.
 17. The electrochemical device as claimed inclaim 14, which is a lithium secondary battery.
 18. The electrochemicaldevice as claimed in claim 14, wherein the separator comprisesheat-absorbing particles which absorb heat energy generated at atemperature (T) higher than a normal drive temperature of anelectrochemical device so that they are pyrolyzed, or consume the heatenergy, as a component for forming or coating the separator.